1. Field of the Invention
The present invention relates to a method of fabricating silica glass by a sol-gel process, and more particularly, to a method of injecting sol into a mold during fabrication of silica glass by a sol-gel process.
2. Description of the Related Art
In general, silica glass is transparent and chemically inert, and has a high level of thermal stability and strength, and a low thermal expansion coefficient. Because of such characteristics, silica glass has been useful for optical devices such as an optical fibers or optical lenses.
Basically, an optical fiber is comprised of a core surrounded by a cladding having a refractive index different from the core such that light is totally reflected from the core. In order to fabricate optical fibers, an optical fiber preform including a core rod and an overcladding tube enclosing the core rod is prepared. Then, the optical fiber preform is thermally-treated and then extended to form the optical fibers.
Hereinafter, a method of manufacturing an overcladding tube formed of silica glass by the sol-gel process will be briefly described. First, silica particles are dispersed in water. Fumed silica is used for the silica particles. The fumed silica is a pyrogenic silica obtained by thermally decomposing a silicon compound, e.g., silicon tetrachloride (SiCl.sub.4). Then, a dispersing agent, a binder and a plasticizer are added to the mixture, and mixed well to form a sol.
The obtained sol is aged for a predetermined period of time. After removing air bubbles from the aged sol, a gelling agent is added and the sol is immediately poured into a mold for gelation. After gelation is completed, the gel is separated from the mold and then dried.
Then, the dried gel is thermally-treated to remove organic substances from the gel. Then, dehydroxylation and sintering processes are performed, thereby completing an overcladding tube formed of silica glass.
Injection of the sol into the mold is achieved by pouring the sol from the top to the bottom of the mold, by a method using the height difference between the sol and a mold having a bar and a tube, and a sol container, or by a method using a pump.
In the method of pouring the sol from the top to the bottom of the mold, air bubbles are formed during dropping of the sol and these bubbles remain in a gel formed from the sol.
The method using the height difference between the sol and the mold, basically requires a smooth up-and-down movement of the sol container, so that the design of such a sol injecting device is correspondingly complicated. Also, it is difficult to mold the sol within a short time and to control the sol injecting rate to an intended level.
In addition, the method using a pump causes contamination of the sol by impurity during injection of a sol when the sol is injected rapidly. Thus, it is difficult to obtain a high-purity silica glass.
Examples of the silica sol-gel molding from the conventional art are seen, for example, in the following U.S. Patents. U.S. Pat. No. 4,574,063, to Scherer, entitled Method Of Forming Glass Or Ceramic Article, describes a method for casting a silica sol-gel in a mold. In one example given, the suspension is transferred by gravity from a separatory funnel through a 1/8" Teflon(R) plastic tube, prior to addition of the gelling agent. In another example, the suspension is drawn from a separatory funnel through a plastic tube to a glass delivery cylinder by means of a motor-driven plastic piston in the cylinder, and then is forced by the piston into the mold through a flexible plastic tube and a stainless steel delivery tube. These transfer methods are similar to the height difference and pump methods described above.
U.S. Pat. No. 5,169,421, to Yagi et al., entitled Method Of Manufacturing Silica Glass Optical Wavelength Preform, describes an extruding apparatus for making a silica glass preform from a plastic silica suspension. This method does not involve sol-gels, and the silica gel suspension used is much more viscous than an ungelled sol. The apparatus is essentially a mechanical pump.
U.S. Pat. No. 5,240,488, to Chandross et al., entitled Manufacture Of Vitreous Silica Product Via A Sol-Gel Process Using A Polymer Additive, describes a sol-gel process involving a particular additive to the sol. In the examples given, the sol is generally described as poured, cast or transferred to a mold. In some cases, the free space in the mold was evacuated to rid the sol of trapped air after the sol was poured.
U.S. Pat. No. 5,578,101, to Blonder et al., entitled Method of Making a Sol-Gel Glass Body And Removing Same From Mold, describes an electrochemical process for releasing a sol-gel from a mold. In the examples given, the sol-gel is poured into the mold.
Based on our observation of the art, then, I have decided that what is needed is an improved method for transferring a silica sol to a mold, allowing rapid, controlled transfer without formation of air bubbles or contamination of the sol.